Process for coating a metal substrate for packaging end use

ABSTRACT

A process for coating a metal substrate for packaging end use by applying to a primary substrate a thermosettable composition which is free of added solvent and has a residual solvent content not exceeding 10% derived from the manufacture of any of the ingredients. The composition is applied as a thermosettable film by extrusion in melt (including plastified) form through an extrusion coating die onto a substrate. There is relative movement between the die and the substrate so that successive areas on the substrate are coated with the thermosettable composition, so as to form a film. The primary substrate is the metal substrate for packaging end use or an intermediate temporary support from which the applied coating is transferred onto the metal substrate for packaging end use. The applied coating is thermally cured after extrusion through the extrusion coating die. Compositions suitable for use in this and other coating processes are described.

This is a nation stage application of PCT/GB93/01418, filed Jul. 6,1993.

This invention relates to a process for forming a cured thermosetcoating on a metal substrate for packaging end uses, including packagingof liquid and dry products, especially for use in the packaging(including processing or holding) of foods and beverages. The inventionalso relates to thermosettable compositions for use in coating metalsubstrates for packaging end use.

BACKGROUND OF THE INVENTION

Currently, thermosetting protective and decorative coatings aregenerally applied to metal strip or sheet for packaging end uses byroller-coating of a solvent-based lacquer (which may comprise water anda co-solvent) typically comprising 30-60% by weight solids. Afterapplication of the lacquer to the metal strip or sheet, the solvent isremoved by evaporation and the applied coating is cured. For cost andenvironmental reasons, it would be desirable to be able to reduce theusage of organic solvents in the application of thermosetting coatings,and the present invention is directed to that objective.

U.S. Patent Specification No. 4,990,364 describes solvent-free, lowmonomer or monomer-free polymerisable or curable compositions for use incoating various classes of substrates, including metal substrates forpackaging end use. The compositions are indefinitely stable under theconditions described and are curable only by means of free-radicalinitiators in conjunction with heat treatment; by means of actiniclight, especially ultra-violet radiation; or by electron radiation.There is no disclosure of thermally activated crosslinkable systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an apparatus, shown partly in section and partly indiagrammatic form, for carrying out the process of the invention; and

FIG. 2 is another form of apparatus, shown in diagrammatic form,suitable for carrying out the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for coating a metal substratefor packaging end use, which comprises applying to a primary substrate athermosettable (that is to say, thermally crosslinkable/curable)composition suitable for forming a coating for packaging end use,wherein the thermosettable composition is substantially solvent-free,the composition is applied as a thermosettable film by extrusion in melt(including plastified) form through an extrusion coating die onto asubstrate, there being relative movement between the die and thesubstrate so that successive areas on the substrate are coated with thethermosettable composition so as to form a film, the primary substratebeing the metal substrate for packaging end use or an intermediatetemporary support from which the applied coating is transferred onto themetal substrate for packaging end use, and the applied coating isthermally cured after extrusion through the extrusion coating die.

It is an important feature of the present invention that the solventcontent of the applied composition is to be low as compared with thepractice hitherto in applying thermosetting coatings for packagingpurposes. Specifically, no solvent is added to the composition at anystage of the process and, moreover, the residual solvent content of thecomposition derived from the manufacture of any of the ingredients doesnot exceed 10% by weight, advantageously does not exceed 5% by weight,preferably does not exceed 4% or 3% by weight, and more especially doesnot exceed 2% by weight. In many cases, the solvent content of thecomposition will be sufficiently low that it will not be necessary totake any further measures to comply with legislation concerning volatileorganic solvent emissions.

It is also an important feature of the process of the invention that itis possible to obtain uniform thin films on the substrate by extrusionof substantially solvent-free thermosettable compositions in molten orplastified form.

Metal substrates coated in accordance with the process of the inventionmay be employed for any of a wide range of packaging end uses, and acoating may be applied in accordance with the invention for interiorprotection of containers and the like and/or for exterior decoration.Thus, for example, the process may be used to obtain coated substratesfor the packaging of aerosol preparations or paints. More especially,however, the process is used to obtain coated metal substrates which aresuitable for use in the packaging (including processing or holding) offoods and beverages. For that purpose the coated substrate may be formedinto a container for food or drink, or into a component for such acontainer, or into a closure for such a container, such as a lid orbottle top. The process may be used to apply an exterior decorativecoating and/or an interior protective coating. For internal protectivepurposes it will be appreciated that the finished coating must be suchas to provide a surface which will not contaminate foods or beveragescoming into contact therewith.

The invention also provides a solvent-free, thermosettable compositionfor use in coating metal substrates for packaging end use. Suchcompositions may be applied to metal substrates by means of the processof the present invention or by other suitable coating processes, forexample, by roller coating or by solid block coating techniques.

It will be appreciated that the term "thermosettable", as used herein inrelation to compositions, is used in its ordinary meaning to denote thatthe composition is curable by the action of heat, as distinct fromcompositions which, for curing, require additional or alternativeexpedients such as the presence of free-radical initiators,photo-initiators, or the use of electron radiation. It is a feature ofthe present invention that the coating composition is thermosettable andcomprises components which are thermally reactive but which arenevertheless manipulated in melt form prior to application to thesubstrate and curing. In contrast, for instance, with U.S. Pat. No.4,990,364, compositions of the present invention do not require thepresence of ethylenically unsaturated reactive groups such as, forexample, acrylic and/or methacrylic groups, prior to curing, andpreferred thermosettable compositions in accordance with the inventioncontain substantially no such reactive unsaturation.

A solvent-free thermosettable composition in accordance with theinvention will generally comprise a substantially solvent-freefilm-forming resin and a corresponding curing agent (which may itself beanother film-forming polymer). Self-crosslinkable systems, not requiringany added curing agent, are possible in principle.

The composition may, for example, comprise one of the following systems,or a blend of two or more such systems may be used:

1) Epoxy resin, or epoxy novolac resin, preferably, but not limited to,bisphenol A/epichlorohydrin-based materials exemplified by solidmaterials such as, for instance, Shell Epikote 1009, 1007, 1004, 1002,1001 and 828, in combination with one or more resole phenolic resinswhich may be alkylated or non-alkylated as exemplified by Uravar FB190,Uravar FB120, Varcum 29-101, Varcum 29-108, Varcum 29-159, Varcum 29-183and Varcum 94-635.

The epoxy resin may be modified with (a) one or more polyesters, whichmay be branched or linear, acid- and/or hydroxy-functional, asexemplified by Dynapol LH820 (a saturated, medium molecular weight,linear, hydroxy-functional polyester); Dynapol L858 (a saturated, highmolecular weight, branched polyester); Dynapol L206, (a saturated, highmolecular weight, linear polyester); and Uralac 2695 (a saturated,medium molecular weight, branched carboxylated polyester); or (b) one ormore diacids, exemplified by succinic, adipic, sebacic, isophthalic,terephthalic and phthalic acids or (c) one or more diols exemplified byethylene, propylene, diethylene and triethylene glycols, 1,4-butanediol,1,6-hexanediol, and cyclohexanedimethanol.

The system may be further crosslinked with one or more essentiallysolvent-free aminoplast resins, exemplified by Cymel 301(hexamethoxymethyl melamine), Cymel 1123 (Benzoguanamine formaldehyde),Cymel 1170 (Glycol uril formaldehyde) and UFR-80 (Urea formaldehyde),all from Dyno Cynamid, and/or bisphenol A epichlorohydrin-based epoxyresins, exemplified by liquid materials such as, for instance, ShellEpikote 828, Dow DER330, Ciba Araldite GY2600 and 260, and/or epoxynovolac resins exemplified by Dow DEN 431 or DEN 438.

Polyester resin, as exemplified by those described in 1)a) above, incombination with one or more resole phenolic resins as exemplified bythose described in 1) above, and/or essentially solvent-free aminoplastresins, as exemplified by those described in 1) above. The system may befurther crosslinked with additions of epoxy (e.g. liquid epoxy) and/orepoxy novolac resins as exemplified by those described in 1) above.

3) Epoxy resin, as exemplified by materials such as those describedin 1) above, in combination with one or more organic anhydrides oranhydride oligomers as exemplified by one or more of trimellitic,succinic, phthalic, hexahydrophthalic and tetrahydrophthalic anhydrides.

4) Epoxy resin, as exemplified by materials such as those describedin 1) above, in combination with high solids, typically at least 60%,acid-functional acrylic polymers, acid value typically greater than 50mg KOH/g as exemplified by Reichold Synthemal 40-462, McWhorterAcrylamac 7555, HiTek CMD 979, HiTek RGX-87425, Paraloid AT-70, orParaloid AT-85.

5) Hydroxy-functional polyester, as exemplified by those described in 1)above, in combination with isocyanate which may be internally orexternally blocked as exemplified by uret diones, phenol blockedisocyanates, such as for instance, Desmodur AP stable (Bayer), and/oressentially solvent-free aminoplast resin as described in 1) above.

6) (a) Emulsion polymers based on crosslinkable thermosetting acrylicresins, as exemplified by Union Carbide Ucar 4510 and Rohm & Haas PrimalAC1822, crosslinked with additional components such as essentiallysolvent-free resole phenolic resins, essentially solvent-free aminoplastresins, liquid or solid epoxy resins or epoxy-novolac resins, all asdescribed in 1) above.

(b) Emulsion polymers based on self-crosslinking thermosetting acrylicresins, as exemplified by Rhoplex AC-604, AC-625, AC 1230 and HA-16.

7) A linear or branched polyester with acid functionality sufficient forreaction with the chosen crosslinking agent, which may be an epoxyresin, more especially an epoxy resin as described in 1) above.Typically, the polyester will have an acid value of at least 30 mg KOH/gas exemplified by EMS Grilesta V72/6 and Uralac P2695.

The system may be crosslinked with dicyandiamide, epoxy-novolac,phenolic and/or aminoplast resins as described in 1) and 2) above.

It will be understood that in principle a coating composition accordingto the invention may comprise one or more film-forming resins, which maybe self-crosslinkable or may be used in conjunction with one or morecuring agents as appropriate, depending upon the number and type ofcurable functional groups in the or each film-former. It is a feature ofthe invention, however, that it is not in general necessary to use amixture (or alloy) of film-forming resins to achieve the desiredcharacteristics, for example, thickness, in the applied coating aftercuring, and mention should therefore be made of compositions whichcomprise only one film-forming resin (which may be a self-crosslinkablematerial or may be used with one or more appropriate curing agents ifrequired), i.e. a single such resin.

It is an important feature of the present invention that thermosettablecompositions with a wide range of initial physical and chemicalcharacteristics can be utilised. Thus, for example, compositions for usein accordance with the present invention may use polymeric materialswith Tg, molecular weights and curing chemistries which would not beconsidered useful for powder coating compositions. More especially,certain specific forms of thermosettable compositions in accordance withthe invention may usefully be characterised as follows:

(a) Compositions which in uncured condition have a Tg of

below 40° C., preferably 35° C. or below, and more especially 30° C. orbelow, with mention also being made of compositions having a Tg of 5° C.or below, including such compositions having a Tg of below 0° C.;

(b) Compositions which in uncured condition have a Tg of above 70° C.,preferably above 85° C., and more especially of 90° C. or above; and

(c) Compositions which cure by condensation reactions yielding volatileby-products (for example, compositions comprising phenol-formaldehyde ormelamineformaldehyde condensates, whether as the sole film-formingpolymer system or as a crosslinker for other polymer systems, forexample, epoxy or polyester systems).

All Tg values given in the present specification refer to such values asdetermined by differential scanning calorimetry (D.S.C.).

Compositions (a) to (c) above may be applied to metal substrates by anysuitable process, and especially by a melt extrusion process inaccordance with the present invention. It is to be understood thatcompositions which, in uncured condition, have Tg in the range of from30° C. to 85° C. (preferably from 40° C. to 70° C.), and compositionswhich do not cure by condensation reactions yielding volatileby-products, are disclosed in accordance with the present invention forapplication to metal substrates by processes other than powder coatingprocesses and, especially, for application by melt extrusion processesin accordance with the present invention.

In the case where the coating is to provide an interior protectionsurface in food or beverage packaging, it will be appreciated that thecomponents of the coating composition must be recognised as food contactapprovable for use in food contact surfaces of articles for packaging,processing or holding of food and beverages.

The proportion of curing agent in a coating composition for use inaccordance with the invention may be in the range of from 0% by weight(for a self-crosslinking system) to 50% by weight, and will usually bein the range of from 5 or 10 to 25% by weight, based on the total weightof the composition.

The number average molecular weight of the or each film-forming polymerin the composition before curing will in general be in the range of from300 to 250000, more usually from 300 to 25000, advantageously from 300to 7000, 8000 or 10000, preferably not exceeding 5000, more especiallyfrom 500-5000.

Where appropriate, a coating composition for use in accordance with theinvention may include a quantity of catalyst for the curing reaction.Examples of suitable catalysts include salts of strong or weak acidssuch as zinc stearate, (dialkyl) tin dialkanoates, for instance,dibutyltin dilaurate, blocked acid phosphates and blocked substitutedbenzene sulphonic acids, triphenyl-phosphine, phosphonium halides, andtriethylamine.

To achieve decorative and other advantageous effects, such as corrosioninhibition or control of thermomechanical properties, inorganicpigments, typically titanium dioxide, may be dispersed in the majorfilm-forming component, or as a predispersed paste, at up to 60% byweight of the total composition. Incorporation of other pigmentation ispossible by the same techniques.

It will be appreciated that for certain packaging applications, nopigmentation will be required, and the invention accordingly alsoprovides unpigmented systems.

The film formation properties and/or the dry film properties of thefinal cured film may be enhanced by the incorporation of inorganic orpolymeric fillers, extenders and additives up to a content of, forexample, 30% by weight of the total film composition. Such fillers,extenders and additives may contribute, for example, to the flow,lubricity, flexibility, adhesion, film formation and stability of thefinal composition, or to combinations of those properties. The materialsmay be incorporated in the composition in the same manner as describedabove for pigments or in appropriate cases may be incorporated bycomelting.

A metal substrate for packaging end use may comprise aluminium, tinplateor steel (which may be tin-free steel-ECCS). The thickness of thesubstrate will be selected as appropriate for the particular packagingend use, and may be in the range from 0.05 mm to 3 mm, typically in therange from 0.05 mm to 2 mm, and advantageously in the range 0.1 mm to0.4 mm.

The metal substrate may be subjected to appropriate pre-treatment, suchas flame treatment or corona discharge, or an initial conversion coatingmay be applied, such as phosphate or chromate treatment (for example,the chromate treatment ALOCROM 404).

The metal substrate may be in the form of a moving strip or sheet, andmay be transported continuously past the extrusion coating die orcoating transfer station, as the case may be. Instead, the metalsubstrate may be in the form of a tube, preferably an open cylindricaltube, and the extrusion coating die may extend circumferentially aroundor within the tube over the whole or part of its circumference. Althoughthe normal arrangement will be for the extrusion coating die to remainstationary and for the substrate to be transported past it, it ispossible in principle for the substrate to remain stationary and for thedie to be moved along it. As a further possibility, a substrate intubular form may be rotated around a stationary extrusion coating diearranged internally within the tube or externally around the tube.

In the case in which the primary substrate is an intermediate temporarysupport from which the applied coating is transferred to the metalsubstrate for packaging purposes, the temporary substrate may comprise asiliconised paper from which the applied coating can readily bereleased, or may comprise another low surface energy substrate, forexample, a fluoropolymer impregnated support such as, for instanceTygaflor.

In the process of the invention the coating composition is supplied tothe extrusion coating die in melt (or plastified) form. Melting (orplastification) of the composition may be carried out in any suitablemelt-mixing apparatus, which may be a static or dynamic mixer, forexample, a Banbury mixer or a Z-blade mixer. The melt-mixed compositionmay be supplied to the extrusion coating die by means of a suitablepump, such as a gear pump or other positive displacement pump. Anextruder may be used as a melt-mixer and a pump, or may be used only asa pump for a composition which has been melt-mixed by some other means.

In another form of process, the film-forming polymer and the curingagent are each independently metered to a mixer located immediatelyupstream of the extrusion coating die. Examples of mixers which may beused in such a process include high-efficiency mixers such as static orcavity-transfer mixers. Thus, for example, the film-forming polymer andthe curing agent can each be fed through a separate melt hopper into arespective gear pump, or other positive displacement pump, which in turnfeeds the corresponding component to a mixer located immediatelyupstream of the extrusion coating die. Such a process has the advantageof preventing or reducing unnecessary premature contact betweenco-reactive components of the composition. In industrial-scaleprocessing, bulk low-temperature storage of components, fed from storageto a relatively small extruder to melt-mix and pump the compositionthrough the coating die, may offer advantages (as compared with the useof large melt hoppers) in minimising losses of materials in the event ofan enforced shutdown of the system.

A catalyst for the curing reaction, if used, may be supplied inadmixture with one or other of the coreactive components of thecomposition, or may be injected directly into the compositionimmediately upstream of the extrusion coating die.

The thickness and quality of the coating applied to the substrate aredependent primarily on the following parameters:

1) The temperature of the substrate before application of the coating.In general, a metal substrate is heated to a temperature in the range offrom 50° to 250° C., preferably from 70° to 200° C., for example, from100° to 200° C., before application of the coating composition. Suchheating will generally facilitate the production of relatively thincoatings. It is further believed that the preferred temperature of thesubstrate is related to the glass transition temperature (Tg) of theuncured composition, and is preferably in the range of from Tg+10° C. toTg+200° C., more especially from Tg+25° C. to Tg+150° C. For theavoidance of doubt, the expression "Tg+10° C." means a temperature thatis 10° C. above the Tg value, and similar expressions herein are to beunderstood accordingly.

2) The temperature of the extrusion coating die, which may in general bein the range of from 50° to 200° C., preferably from 80° to 180° C.,advantageously at least 100° C., for example, from 120° to 180° C. It isfurther believed that the preferred temperature of the extrusion coatingdie is related to the glass transition temperature (Tg) of the uncuredcomposition, and is preferably in the range of from Tg+10° C. to Tg+200°C., more especially from Tg+25° C. to Tg+150° C.

3) The temperature of the molten composition immediately prior toapplication to the substrate, which may in general be in the range offrom 50° to 200° C., preferably from 80° to 180° C., advantageously atleast 100° C., for example, from 120° to 180° C. It is further believedthat the preferred temperature of the composition immediately prior toapplication to the substrate is related to the glass transitiontemperature (Tg) of the uncured composition, and is preferably in therange of from Tg+10° C. to Tg+200° C., more especially from Tg+25° C. toTg+150° C.

It will be appreciated, as a feature of preferred forms of processaccording to the invention, that neither the temperature of theextrusion coating die [parameter 2) above], nor the temperature of themolten composition immediately prior to application [parameter 3)above], should differ unduly from the temperature of the substrate priorto the application of the coating [parameter 1) above]. Thus, forexample, those temperature differences may be ±50° C., and moreespecially ±30° C. Particular mention may be made of processes in whichthe temperature of the extrusion coating die, and of the compositionimmediately before application, are each greater than the temperature ofthe substrate.

4) The rate of relative movement between the die and the substrate,which may be in the range of from 1 to 300 meters/minute, advantageouslyfrom 5 to 200 meters/minute, for example, from 10 to 150 meters/minute,especially at least 50 meters/minute.

5) The distance between the substrate surface and the outlet of theextrusion coating die, which may be in the range of from 2 to 150microns, advantageously from 2 to 100 microns, preferably from 2 to 50microns, more especially from 2 to 30 microns. Typically, the outlet ofthe extrusion coating die will comprise a rectangular slot bounded oneach of its long sides by a flat end plate. In such an arrangement, ithas been found to be advantageous to arrange the die so that the gapbetween the die face and the substrate is of generally wedge-shapedcross-section, with the gap narrowing progressively in the direction ofmotion of the substrate. The figures given above for the distancebetween the outlet of the extrusion coating die and the substrate arethen applicable to the shortest distance between the die and thesubstrate.

6) The outlet gap width of the extrusion coating die, which may be inthe range of from 50 or 100 to 1500 or 3000 microns, typically in therange of from 400 to 1000 microns.

7) The viscosity of the coating composition at the point of applicationto the substrate. In general, the viscosity at the point of applicationwill not exceed 1000 poise, advantageously does not exceed 500 poise,preferably does not exceed 100 poise, and more especially does notexceed 50 poise (as measured at the application temperature by a coneand plate viscometer such as that supplied by Imperial ChemicalIndustries).

8) The rate at which the coating composition is applied to thesubstrate, which may, for example, be in the range of from 2 to 9000 cm³per minute for a strip width of 1 meter.

The residence time of the composition at the application temperatureshould be kept as low as possible, so as to minimise premature curing ofthe composition, and should in particular be well below the gel time ofthe composition at that temperature. It will be appreciated that, inconsidering what will be an acceptable residence time of the compositionat the application temperature, it is also necessary to take intoaccount the thermal history of the composition prior to application,i.e. the average residence time in different parts of the systemmaintained at different temperatures. By way of illustration, especiallyfor compositions which in uncured condition have a relatively high Tg(say, 40° C. or above), the residence time of the material at theapplication temperature is advantageously less than 60 seconds, andpreferably less than 30 seconds, for example, less than 20 seconds, moreespecially less than 10 seconds. For compositions which in uncuredcondition have a relatively lower Tg (for example, below 40° C.) butwith cross-linking chemistries requiring curing at, say, 180°-200° C.for 10-15 minutes, the application temperature can be lower. The geltime of the compositions at such lower temperatures will typically belonger and, in such circumstances, the residence time of the compositionat the application temperature may be longer. Thus, for example, theresidence time at the application temperature may be up to 15 minutes,advantageously up to 10 minutes, preferably up to 5 minutes, forexample, up to 200 seconds, more especially up to 100 seconds.

The outlet orifice of the extrusion coating die will in general be inthe form of a slot of normally rectangular cross-section. The slot mayextend continuously along the length of the die or may be interrupted atintervals by closed portions which, in use, will result in correspondinguncoated portions on the substrate. If desired for special effects, forexample to produce differential film thicknesses, the supply toindividual open portions of the extrusion orifice may be controlled byseparate valves.

In order to distribute the molten composition along the length of thedie, there may, for example, be a supply conduit at each end of the die,or the total melt flow may be distributed through a plurality of supplyconduits arranged to open at different positions along the length of thedie.

To assist in minimising any premature curing of the composition, theextrusion coating die should not contain any zones in which the meltflow may become stagnant.

As already indicated, the distance between the substrate and the outletof the extrusion coating die is one of the parameters influencing thethickness and quality of the coating applied to the substrate. If thedie is positioned too close to the substrate, there is a risk that thecomposition will flow around the side of the die and deposit at theedge. If, on the other hand, the die is too far from the substrate,there is a risk that the substrate will not be completely coated, and a"herringbone" pattern may be produced.

In one form of process according to the invention, the substrate isarranged to pass closely adjacent to the extrusion die outlet and ispassing across or around a backing roller (coated, for example, withrubber) at the point of application of the extruded film. For example,when the substrate is passing around a backing roller from below duringapplication of the coating, the site of application may be at any pointaround the circumference of the backing roller with which the movingsubstrate is in contact. Preferably, the arrangement is such that theaxis of the extrusion die extends along a radius of the backing roller.More particularly, control of application of the coating, in terms ofthickness and appearance of the finished film after curing, isfacilitated if the backing roller is disposed horizontally and the siteof application of the coating composition is in a plane perpendicular tothe axis of rotation of the roller. In such a process, the coatingpressure is controlled by the position of the die in relation to thesubstrate. In the case in which the substrate is passing over or acrossa backing roller at the point of application, good control of coatingthickness and appearance can in general be achieved without it beingnecessary to pass the coated substrate through any subsequent calenderrollers.

In another form of process according to the invention, there is nobacking roller behind the substrate at the point of application of thecoating composition. The coating pressure in such a process iscontrolled by the tension under which the substrate is maintained duringtransport past the extrusion die outlet.

In general, the use of a backing roller at the point of applicationfacilitates the application of relatively thin coatings, and thearrangement with no backing roller may be more suitable for theapplication of thicker coatings.

For thermal curing of the applied composition, the curing temperaturemay be in the range of from 140° to 300° C., depending upon the natureof the composition, and may typically be in the range of from 170° to270° C.

Depending upon the nature of the composition and the method of curing,the curing time may typically be in the range of from 2 seconds to 15minutes, for example, from 2 to 40 seconds (preferably from 10 to 25seconds) for rapid curing, from 2 to 5 minutes for medium-rate curing,or from 5 to 12 minutes for slower curing.

In the case where the coating composition is applied initially to anintermediate temporary support, curing may be effected wholly orpartially whilst the coating is still on the temporary support, or maybe wholly or partly deferred until after the composition has beentransferred to the final substrate.

The use of an intermediate support may be advantageous in certaincircumstances. In particular, it may assist in reducing wear on theextrusion coating die, and in some instances it may be easier to applythe coating composition to an intermediate support (which will ingeneral be a relatively soft material) rather than directly onto thefinal metal substrate. Transfer of the coating from the intermediatesupport onto the final substrate may be carried out by methods analogousto those used in conventional lamination technology, for example, usinghot nip rollers.

For packaging end use, the thickness of the applied coating after curingwill in general be 30 microns or less, advantageously less than 25microns, preferably less than 20 microns for example, less than 18, 16,14, 12 or 10 microns, and is more particularly in the range of from 2 to10 microns, for example from 4, 6 or 8 to 10 microns. Typically, thethickness of a cured coating in accordance with the invention will be inthe range of from 4 to 12 microns, which will generally correspond to afilm weight of from about 4 to 16 g/m².

After curing, the coated metal substrate may be treated with one or morelubricants or waxes, or may be subjected to any other appropriatepost-curing treatment.

The invention also provides a container, more especially for food ordrink, or a component or closure for such a container, formed from ametal substrate that has been coated by the process of the invention.

Formulation Examples

The following Examples illustrate the formulation of thermallycrosslinkable compositions for use in the coating process of theinvention. It will be appreciated that the following formulationinformation relates to the essential components of the compositions ofthe Examples and that, as already described hereinbefore, coatingcompositions according to the invention will generally also include oneor more fillers, extenders and/or performance additives.

Formulation Example 1

800 g of Epikote 1004 (Bisphenol A based epoxy resin from Shell) wascharged to a 2 liter flask fitted with a stirrer. The resin was heateduntil molten and the temperature raised to 120°-140° C. 200 g of UravarFB190 (solid phenolic resin from DSM) was added quickly to the flask andallowed to mix in. The contents of the flask were discharged onto awater-cooled tray in order to cool the resin blend quickly. The solidresin blend was then broken up for use in an extruder. The hot-plate geltime of the mixture at 185° C. was 150 seconds, and the Tg was 40° C.

Formulation Example 2

A polyester with a theoretical Mn of 1550 containing propylene glycol(44.00 moles), trimethylol propane (8.40 moles), terephthalic acid(31.70 moles) and isophthalic acid (15.85 moles) was prepared. The resinhad a hydroxyl value of 105 mgKOH/g, an acid value of 11 mgKOH/g and aTg of 54° C.

800 g of this polyester was charged to a 2 liter flask fitted with astirrer. The resin was heated until molten and the temperature raised to120°-140° C. 200 g of Uravar FB190 (solid phenolic resin from DSM) wasadded quickly to the flask and allowed to mix in. The contents of theflask were discharged onto a water cooled tray in order to cool theresin blend quickly. The solid resin blend was then broken up for use inan extruder. The hot-plate gel time of the mixture at 185° C. was 150seconds, and the Tg was 45° C.

Formulation Example 2.1

A polyester with a theoretical Mn of 1550 containing propylene glycol(44.0 moles), trimethylol propane (8.4 moles), isophthalic acid (24.4moles) terephthalic acid (4.3 moles) and adipic acid (18.9 moles) wasprepared. The resin had a hydroxyl value of 105 mgKOH/g, an acid valueof 11 mgKOH/g and a Tg of 5° C. The resin was frozen then broken up foruse in an extruder. 800 g of this polyester was charged to a 2 literflask fitted with a stirrer. The resin was heated until molten and thetemperature raised to 120°-140° C. 200 g of Uravar FB190 (solid phenolicresin from DSM) was added quickly to the flask and allowed to mix in.The contents of the flask were discharged onto a refrigerated tray inorder to cool the resin blend quickly. The hot-plate gel time of themixture at 185° C. was 150 seconds, and the Tg was -6° C. The resinblend was frozen then broken up for use in an extruder.

Formulation Example 3

Epikote 1007, a bisphenol A resin from Shell, (5400 g) was dissolved inbutyl acetate (4154 g) at 95° C. in a reactor fitted with a stirrer. Tothe solution was added succinic anhydride (350 g) and the whole wasprocessed at 95° C. until most of the anhydride had reacted. Thesolution was reduced to 50% NVC with butyl acetate.

This modified epoxy resin solution (100 parts) was dissolved in butanone(100 parts). The resulting solution was spray dried at near ambienttemperatures using a laboratory spray drier. The hot-plate gel time ofthe mixture at 185° C. was 50 seconds, and the Tg was 58° C.

Formulation Example 4

Paraloid AT-70, a carboxy-functional acrylic resin from Rohm & Haas, wasspray dried at elevated temperatures using a laboratory spray drier toproduce an essentially solvent-free white powder.

800 g of Epikote 1004 (Bisphenol A based epoxy resin from Shell) wascharged to a 2 liter flask fitted with a stirrer. The resin was heateduntil molten and the temperature raised to 120°-140° C. 200 g of thespray dried acrylic resin above was added. The resins were held at120°-140° C. and allowed to mix in. The contents of the flask weredischarged onto a water-cooled tray in order to cool the resin blendquickly. The solid resin blend (Tg: 56° C.) was then broken up for usein an extruder.

Formulation Example 4.1

Paraloid AT-70 a carboxyl functional acrylic resin from Rohm & Haas wasspray dried at elevated temperatures using a laboratory spray drier toproduce an essentially solvent free white powder.

500 g of Epikote 1004 (Bisphenol A based epoxy resin from Shell) wascharged to a 2 liter flask fitted with a stirrer. The resin was heateduntil molten and the temperature raised to 120°-140° C. 500 g of thespray dried acrylic resin above was added. The resins were held at ca120°-140° C. and allowed to mix in. The contents of the flask wasdischarged onto a water cooled tray in order to cool the resin blendquickly. The solid resin blend (Tg: 45° C.) was then broken up for usein an extruder.

Formulation Example 5

750 g of the polyester used in Example 2 was charged to a 2 liter flaskfitted with a stirrer. The resin was heated until molten and thetemperature raised to 140°-160° C. 525 g of Desmodur AP stable (solidblocked isocyanate resin from Bayer) was added quickly to the flask andallowed to mix in. The contents of the flask were discharged onto awater-cooled tray in order to cool the resin blend quickly. The solidresin blend (Tg: 45° C.) was then broken up for use in an extruder.

Formulation Example 5.1

750 g of the polyester used in Example 2.1 was charged to a 2 literflask fitted with a stirrer. The resin was heated until molten and thetemperature raised to 120°-130° C. 525 g of Desmodur AP stable (solidblocked isocyanate resin from Bayer) was added quickly to the flask andallowed to mix in. The contents of the flask was discharged onto arefrigerated tray in order to cool the resin blend quickly. The resinblend (Tg: -5° C.) was frozen then broken up for use in an extruder.

Formulation Example 6a

A polyester with a theoretical Mn of 2800 containing neopentyl glycol(0.9 moles), diethylene glycol (0.2 moles), terephthalic acid (0.9moles), isophthalic acid (0.1 moles) and trimellitic anhydride (0.2moles) was prepared using Fascat 2005 (stannous chloride) (0.05% byweight) as catalyst. The resin had an acid value of 78 mgKOH/g, aviscosity of 27 poise at 200° C. and a Tg of 60° C.

A dry mix was formed of Grilonit L1203.5 a Bisphenoi A based epoxy fromEMS, (1285 g), the above polyester (1285 g), Epikote 3003, a Bisphenol Abased epoxy resin from Shell, (175 g), and Uravar FB190 a solid phenolicresin from DSM (194 g).

The mixture was dry blended for 5 minutes until homogeneous and then fedinto an extruder. The extruder was controlled at 80° C. with a feed rateof 10 kg/hr.

The extrudate was rolled on a cooling plate to produce thin sheets priorto kibbling into 0.3 cm³ lumps (Tg: 53° C.)

Formulation Example 6a.1

A polyester with a theoretical Mn of 2800 containing neopentyl glycol(4.37 moles), diethylene glycol (0.97 moles), terephthalic acid (1.09moles), isophthalic acid (0.48 moles), adipic acid (3.27 moles) andtrimellitic anhydride (0.97 moles) was prepared using Fascat 2005(stannous chloride) (0.5% by weight) as catalyst. The resin had an acidvalue of 88 mgKOH/g a viscosity of 11 poise at 140° C. and a Tg of 20°C. The resin was frozen then broken up for use in an extruder.

A dry mix was formed of Grilonit L1203.5 a Bisphenol A based epoxy fromEMS (1285 g), the above polyester (1285 g), Epikote 3003 a Bisphenol Abased epoxy resin from Shell (175 g), and Uravar FB190 a solid phenolicresin from DSM (194 g).

The mixture was dry blended for 5 minutes until homogeneous and then fedinto an extruder. The extruder was controlled at 80° C. with a feed rateof 10 kg/hr.

The extrudate was rolled on a cooling plate to produce thin sheets priorto kibbling into 0.3 m³ lumps (Tg: 1° C.).

Formulation Example 6b

A dry mix was formed of Grilonit L1203.5, a Bisphenol A based epoxy fromEMS (1166 g), the polyester as in Example 6a (1373 g), Tiona 472, atitanium dioxide pigment from SCM (810 g), DEN 438, an epoxy novolacfrom Dow (207 g), and Epikote 3003, a Bisphenol A based epoxy resin fromShell (186 g).

The mixture was dry blended for 5 minutes until homogeneous and then fedinto an extruder. The extruder was controlled at 80° C. with a feed rateof 10 kg/hr.

The extrudate was rolled on a cooling plate to produce thin sheets priorto kibbling into 0.3 cm³ lumps (Tg: 53° C.).

Formulation Example 6b.1

A Masterbatch was prepared by blending together in a coffee grinderGrilonit L1203.5 a bisphenol A based epoxy from EMS (166 g) and AralditeHY960 catalyst from Ciba Geigy (8.0 g).

A dry mix was then prepared of Grilonit L1203.5 (1000 g), Masterbatch asabove (174 g), Polyester as in Example 6a (1373 g), Tiona 472 a titaniumdioxide pigment from SCM (810 g), DEN 438 an epoxy novolac from Dow (207g), and Epikote 3003 a Bisphenol A based epoxy resin from Shell (186 g).

The mixture was dry blended for 5 minutes until homogenous and then fedinto an extruder. The extruder was controlled at 80° C. with a feed rateof 10 kg/hr.

The extrudate was rolled on a cooling plate to produce thin sheets priorto kibbling into 0.3 cm³ lumps (Tg: 1° C.).

One form of apparatus suitable for carrying out the process of theinvention, and a Process Example carried out using the apparatus, willnow be described, by way of example, with reference to FIG. 1 of theaccompanying drawings which shows the apparatus partly in section andpartly in diagrammatic form.

Referring to FIG. 1, a substrate (1) to be coated is drawn across arubber-coated backing roller (2) in the direction of the arrow (3). Thesubstrate is pre-heated by means of a hot air stream from a heater (4).Alternative means of heating a substrate in accordance with theinvention include, for example, heated rollers and induction heating.

The components of a thermosetting coating composition are pre-mixed andmetered into an extruder (5) by way of a hopper (6). Successive sectionsalong the barrel of the extruder are maintained at progressively highertemperatures (T₁ to T₄, respectively). The extruder serves to melt thecomposition and pump the melt along a heated conduit (7) through aheated extrusion coating die (8), from which the molten composition isapplied to the substrate (1) to form a coating (9).

The conduit (7) is maintained at temperature T₅ and the extrusioncoating die (8) is maintained at temperature T₆.

As examples of suitable dimensions, the internal diameter b of theextrusion barrel may be 2.5 cm; the length of the section c of thebarrel may be 62.5 cm; and the length a of the conduit (7) may be 1meter.

Optionally after intervening rolling (by rollers not shown in FIG. 1)the applied composition is cured by passing the coated substrate througha curing oven (not shown).

The substrate bearing the cured coating is fabricated, for example, intofood or beverage cans.

process Example A

Substrates of tinplate or tin-free steel (thickness 0.17-0.22 mm) arecoated with various coating compositions in apparatus as shown inFIG. 1. The corresponding process temperatures, substrate speed andcoating thickness are shown in Table 1, all temperatures being in °C.Each applied coating is cured for 10 minutes at 200° C.

                                      TABLE 1                                     __________________________________________________________________________                      Temperature °C.                                                                         Die ±                                                                          Substrate ±                                                                       Substrate                                                                          Coating                    FORMULATION EXAMPLE                                                                             Tg Extruder zone ± 3° C.                                                             5° C.                                                                      10° C.                                                                        speed                                                                              Thickness                  CHEMISTRY      No.                                                                              °C.                                                                       T.sub.1                                                                         T.sub.2                                                                          T.sub.3                                                                          T.sub.4                                                                          T.sub.5                                                                          T.sub.6                                                                           T.sub.7                                                                              m/min                                                                              Microns                    __________________________________________________________________________    Epoxy/Phenolic 1  40 30                                                                              50 95 95 100                                                                              120  85    7     30                        Polyester/Phenolic                                                                           2  45 30                                                                              50 95 95 100                                                                              160 150    3    <15                        Polyester/Phenolic                                                                           2.1                                                                              -6 10                                                                              25 50 50  50                                                                               70  70    7    10-30                      Epoxy/Anhydride                                                                              3  58 30                                                                              50 95 95 100                                                                              130 150    3    <15                        Epoxy/Acrylate 4  56 30                                                                              50 95 95 100                                                                              120 150    3    <15                        Epoxy/Acrylate 4.1                                                                              45 30                                                                              50 85 85  90                                                                              100 100    3    <15                        Polyester/Isocyanate                                                                         5  45 30                                                                              50 95 95 100                                                                              160 150    3    <15                        Polyester/Isocyanate                                                                         5.1                                                                              -5 10                                                                              25 50 50  50                                                                               70  70    7    10-30                      Epoxy/Polyester                                                                              6a 53 30                                                                              100                                                                              100                                                                              100                                                                              100                                                                              170 150    3    10-20                      Epoxy/Polyester                                                                              6a.1                                                                              1 10                                                                              25 50 50  50                                                                               70  70    7    10-30                      Epoxy/Polyester (Pigmented)                                                                  6b 53 30                                                                              100                                                                              100                                                                              100                                                                              100                                                                              170 150    3    10-30                      Epoxy/Polyester (Pigmented)                                                                  6b.1                                                                              1 10                                                                              25 50 50  50                                                                               70  70    7    10-30                      __________________________________________________________________________

As an illustration of the internal protection performance of thecoatings applied in accordance with the foregoing Process Examples, afood can end is stamped out of each coated substrate and is subjected tostandard adhesion, flexibility, porosity and accelerated pack tests. Theperformance is comparable to that of a commercial food can endroller-coated with conventional solvent-based material.

Another form of apparatus suitable for carrying out the process of theinvention, and a process carried out using the apparatus, will now bedescribed, by way of example, with reference to FIG. 2 of theaccompanying drawings, which shows the apparatus in diagrammatic form.

Referring to FIG. 2, a substrate (10) to be coated is drawn across arubber-coated backing roller (11) in the direction of the arrow (12).The substrate is pre-heated by means of a hot air stream from a heater(13).

The components of a thermosettable coating composition are pre-mixed andfed into a tank (14) in which the composition is melted.

The melted composition is pumped by way of a gear pump (15) along aheated conduit (16) and through a heated extrusion coating die (17),from which the molten composition is applied to the substrate (10) toform a coating (18).

Optionally after intervening rolling (by rollers not shown in FIG. 2)the applied composition is cured by passing the coated substrate througha curing oven (not shown).

The substrate bearing the cured coating is fabricated, for example, intofood or beverage cans.

process Example B

Using apparatus as shown in FIG. 2, a substrate of tinplate or tin-freesteel is coated with a thermosettable composition prepared as follows:

A polyester with a theoretical Mn of 3000 containing terephthalic acid(14.7 moles), isophthalic acid (6.55 moles), adipic acid (44.1 moles),neopentyl glycol (59 moles), diethylene glycol (13.1 moles), andtrimellitic anhydride (13.1 moles) was prepared. The resin had an acidvalue of 75 mgKOH/g and a Tg of -7° C.

5735 g of this polyester was charged to a 101 reactor fitted with astirrer, and heated until molten (85° C.).

1660 g of Epicote 828 and 77 g of Acronal 4F (an acrylic resin fromMonsanto) was charged to a 51 reactor and heated to 75° C. 530 g ofUravar FB 190 (solid phenolic resin from DSM) were added quickly andallowed to mix in. The resulting mixture was discharged into the 101reactor containing the above polyester resin. After stirring, thecontents of the reactor were discharged onto a water-cooled tray inorder to cool the resin blend quickly. The hot-plate gel time of theblend at 185° C. was 65 seconds. The solid blend was broken up andstored in a refrigerated container.

The temperature conditions are as follows in Process Example B:

    ______________________________________                                                        °C.                                                    ______________________________________                                        Substrate (±10° C.)                                                                   85                                                          Melt-tank (14)    90                                                          Heated conduit (16)                                                                             73                                                          Extrusion die (17)                                                                              92                                                          ______________________________________                                    

Food can ends are stamped out of the coated substrate after curing andsubjected to standard performance tests.

In order to provide a basis for comparison with Process Example B, acommercially-available solvent-based internal food can lacquer isapplied to an identical substrate by roller-coating and, after curing,food can ends are stamped out of the coated substrate and subjected tothe same standard performance tests.

The results of the performance tests are summarised in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                       Comparison                                                                              Invention                                            ______________________________________                                        Non-volatile content                                                                           35% solids  100% solids                                      Wedge bend flexibility                                                                         85%         100%                                             Cure (MEK) rubs (No. of                                                                        34 rubs     42 rubs                                          rubs to remove coating)                                                       Adhesion (adhesive tape, 30                                                                    no loss     no loss                                          min @ 80° C.)                                                          Water sensitivity (30 min @                                                                    none        none                                             80° C.)                                                                Film wt range    5-6 g/m.sup.2                                                                             3-6 g/m.sup.2                                    Appearance       smooth/glossy                                                                             smooth/glossy                                    Pack resistance (Scale: 0 = good 4 = poor)                                    (a) Tomato puree                                                                           Stain   2           3                                                         Attack  1           3                                            (b) Cysteine 11/2%                                                                         Stain   0           0                                                         Attack  0           0                                            (c) Tap water                                                                              Blush   0           0                                                         Attack  0           0                                            (d) Acetic acid 1/2%                                                                       Blush   0           0                                                         Attack  0           0                                            ______________________________________                                    

The results show that the performance of the coating applied inaccordance with the invention is generally similar to that of thecommercially-available solvent-based material used as the basis forcomparison.

Further process Examples

In another form of process according to the invention, suitableespecially for use with compositions of relatively low Tg, a compositionas described in Process Example B was charged into a pressure vesselhaving an upper inlet for compressed air and a lower discharge outletconnected by a tube to an extrusion coating die. In operation, thepressure vessel and the tube were heated to 50° C. and the vessel waspressurised to 80 psi thereby causing the coating composition to flowinto and through the die and onto the substrate.

In a specific form of this process, the coating composition was appliedto a substrate comprising a polyethylene terephthalate (PET) filmpre-coated with a silicone release agent. The resulting coating(thickness 20 microns) was then transferred onto a pre-heated metalsubstrate by compression of the coated PET film and the metal substratebetween nip rollers.

We claim:
 1. A process for coating a metal substrate for packaging enduse, which comprises applying to a primary substrate a thermosettablecomposition suitable for forming a coating for packaging end use,wherein the thermosettable composition is free of added solvent and hasa residual solvent content not exceeding 10% by weight derived from amanufacture of any of ingredients of the thermosettable composition, thecomposition is applied as a thermosettable film by extrusion in meltform including in plastified form, through an extrusion coating die ontoa substrate, there being relative movement between the die and thesubstrate so that successive areas on the substrate are coated with thethermosettable composition, so as to form a film, the primary substratebeing the metal substrate for packaging end use or an intermediatetemporary support from which the applied coating is transferred onto themetal substrate for packaging end use, and the applied coating isthermally cured after extrusion through the extrusion coating die toprovide a coating having a thickness of 30 microns or less.
 2. A processas claimed in claim 1, wherein the thickness of the applied coatingafter curing is less than 25 microns.
 3. A process as claimed in claim1, wherein the thickness of the applied coating after curing is lessthan 20 microns.
 4. A process as claimed in claim 1, wherein thethickness of the applied coating after curing is less than 18 microns.5. A process as claimed in claim 1, wherein the thickness of the appliedcoating after curing is less than 16 microns.
 6. A process as claimed inclaim 1, wherein the thickness of the applied coating after curing isless than 14 microns.
 7. A process as claimed in claim 1, wherein thethickness of the applied coating after curing is less than 12 microns.8. A process as claimed in claim 1, wherein the thickness of the appliedcoating after curing is less than 10 microns.
 9. A process as claimed inclaim 1, wherein the thickness of the applied coating after curing is atleast 2 microns.
 10. A process as claimed in claim 1, wherein thethickness of the applied coating after curing is at least 4 microns. 11.A process as claimed in claim 1, wherein the thickness of the appliedcoating after curing is at least 6 microns.
 12. A process as claimed inclaim 1, wherein the thickness of the applied coating after curing is atleast 8 microns.
 13. A process as claimed in claim 1 wherein thethickness of the substrate is in the range of from 0.05 to 3 min.
 14. Aprocess as claimed in claim 1 wherein the residual solvent content ofthe coating composition derived from the manufacture of any of theingredients does not exceed 5% by weight.
 15. A process as claimed inclaim 1, wherein the residual solvent content of the coating compositionderived from the manufacture of any of the ingredients does not exceed4% by weight.
 16. A process as claimed in claim 1, wherein the residualsolvent content of the coating composition derived from the manufactureof any of the ingredients does not exceed 3% by weight.
 17. A process asclaimed in claim 1, wherein the residual solvent content of the coatingcomposition derived from the manufacture of any of the ingredients doesnot exceed 2% by weight.
 18. A process as claimed in claim 1, wherein ametal substrate is heated to a temperature in the range of from 50° to250° C. before application of the coating composition.
 19. A process asclaimed in claim 1, wherein the extrusion coating die is at atemperature in the range of from 50° to 200° C.
 20. A process as claimedin claim 1 wherein the extrusion coating die is at a temperature in therange of from Tg+10° C. to Tg+200° C., where Tg is the glass transitiontemperature of the uncured composition.
 21. A process as claimed inclaim 1 wherein, immediately prior to application to the substrate, themolten composition is at a temperature in the range of from 50° to 200°C.
 22. A process as claimed in claim 1 wherein, immediately prior toapplication to the substrate, the molten composition is at a temperaturein the range of from Tg+10° C. to Tg+200° C., where Tg is the glasstransition temperature of the uncured composition.
 23. A process asclaimed in claim 1, wherein there is relative movement between the dieand the substrate at a rate in the range of from 1 to 300 meters/minute.24. A process as claimed in claim 1, wherein there is a distance in therange of from 2 to 150 microns between the substrate surface and theextrusion coating die.
 25. A process as claimed in claim 1, wherein theextrusion coating die has an outlet gap width in the range of from 50 to3000 microns.
 26. A process as claimed in claim 1, wherein uponapplication the coating composition has a viscosity not exceeding 1000poise.
 27. A process as claimed in claim 1, wherein the processcomprises maintaining the composition at application temperature for aresidence time which does not exceed 15 minutes.
 28. A process asclaimed in claim 1, wherein the substrate passes across or around abacking roller upon application of the extruded film.
 29. A process asclaimed in claim 1, wherein in uncured condition the thermosettablecomposition has a Tg of below 40° C., where Tg is the glass transitiontemperature of the uncured composition.
 30. A process as claimed inclaim 1, wherein in uncured condition the thermosettable composition hasa Tg of above 70° C., where Tg is the glass transition temperature ofthe uncured composition.
 31. A process as claimed in claim 1, whereinthe thermosettable composition cures by at least one condensationreaction fielding volatile by-products.
 32. A process as claimed inclaim 1, wherein the thermosettable composition comprises athermosettable system selected from the group consisting of:(i) an epoxyresin, or an epoxy novolac resin, in combination with one or more resolephenolic resins; (ii) a polyester resin in combination with one or morephenolic resins and/or aminoplast resins; (iii) an epoxy resin incombination with one or more organic anhydrides or anhydride oligomers;(iv) an epoxy resin in combination with one or more high solids,acid-functional acrylic polymers; (v) a hydroxy-functional polyester incombination with a blocked isocyanate and/or an aminoplast resin; (vi)an emulsion polymer based on a crosslinkable, thermosetting acrylicresin in combination with one or more curing agents; (vii) an emulsionpolymer based on a self-crosslinking thermosetting acrylic resin; and(viii) an acid-functional, linear or branched polyester in combinationwith dicyandiamide or with an epoxy or epoxy-novolac resin, a phenolicresin or an aminoplast resin.
 33. A process as claimed in claim 32,wherein the thermosettable system is an emulsion polymer (vi) and thecuring agent is selected from the group consisting of resole phenolicresins, aminoplast resins, liquid epoxy resins, solid epoxy resins andepoxy novolac resins.
 34. A process as claimed in claim 1, wherein thethermosettable composition is unpigmented.
 35. A process as claimed inclaim 1, wherein the thermosettable composition comprises a singlefilm-forming resin and one or more corresponding curing agents asappropriate.
 36. A process as claimed in claim 1, wherein thethermosettable composition comprises a single, self-crosslinkablefilm-forming resin.
 37. A process as claimed in claim 1, wherein thethermosettable composition contains substantially no ethylenicallyunsaturated reactive groups.